Reducing N2 O emissions when burning nitrogen-containing fuels in fluidized bed reactors

ABSTRACT

The amount of N 2  O emission from a fluidized bed reactor is reduced by adding a hydrogen radical providing additive (e.g. a hydrogen containing fuel such as natural gas or alcohol) to the flue gases discharged from the fluidized bed. Sufficient oxygen is present in the flue gases--either by addition with the additive, or by addition of an excess to the combustion chamber--so that the additive reacts with the oxygen, typically raising the temperature of the flue gases (e.g. from about 700°-900° C. to about 950°-1100° C.) that is from about 1292°-1652° F. to about 1742°-2012° F. so that N 2  O production is reduced about 10-90%. The additive may be injected in or just prior to a cyclone for separating particles from the flue gases, in a gas discharge immediately after the cyclone (e.g. just downstream of a heat exchanger for cooling the flue gases and flattening the velocity profile of the flue gases), just prior to a superheater of a convection section, or in a combustion chamber just prior to a gas turbine.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 07/694,206 filed May 3,1991, now U.S. Pat. No. 5,133,950, which in turn is acontinuation-in-part of application Ser. No. 07/509,373 filed Apr. 17,1990, now U.S. Pat. No. 5,043,150.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method (and apparatus) for reducingthe emissions of nitrous oxides N₂ O to the atmosphere from thecombustion of nitrogen containing fuels or other nitrogen containingcombustible compounds. More particularly, this invention relates to amethod and apparatus for reducing such emissions when combusting solidfuels or the like in fluidized bed reactors.

As is well known, oxides of nitrogen are expelled to the air mainly fromtraffic, energy production (e.g. coal combustion), and waste management.Various oxides of nitrogen are produced during the combustion of mostfuels with air. These nitrogen oxides result either from the oxidationof nitrogen in the air itself at elevated temperatures of combustion orfrom the oxidation of nitrogen contained in the fuel.

Numerous attempts have been made to develop methods which reduce thenitrogen oxide emissions in furnaces. The efforts have especially beentowards the reduction of nitrogen dioxide (NO₂) emissions in flue gases.

Another oxide, nitrous oxide N₂ O, has recently been discovered to beone of the "greenhouse effect gases" that is increasing in theatmosphere and may contribute to global warming. When oxidizing in theupper tropospherical layers nitrous oxides (N₂ O) generate nitric oxideNO which is considered to be one of the most important air pollutants:

    N.sub.2 O+hv=N.sub.2 +O N.sub.2 O+O=2NO

Nitric oxide has a similar effect on the climate as carbon dioxide,potentially increasing the temperature and destroying the ozone layer.

It has been reported that N₂ O emissions are generated in higher degreein combustors with relatively low combustion temperatures such as750°-900° C. At higher temperatures the formation of N₂ O does not seemto be a problem, as the formation of N₂ O is slow and the reduction ofN₂ O to N₂ is high.

Fluidized bed combustors operate at temperature ranges more favorablefor N₂ O formation, than most other types of combustors. N₂ O emissionsfrom circulating and bubbling fluidized bed boilers may be on the levelof 50-200 ppm, higher than desired. The object of this invention is,therefore, to provide a method of reducing the emission of N₂ O bothfrom atmospheric and pressurized circulating or bubbling fluidized bedboilers.

The invention is based on the understanding of the kinetics of theformation and destruction of N₂ O. It has been suggested, that HCN,which can be formed from volatile nitrogen or char nitrogen, is themajor precursor of N₂ O formation in combustors, and that N₂ O reductionis strongly dependent on the temperature or H radical concentration. Theincrease in temperature or H radical concentration promotes N₂ Oreduction via the reaction

    N.sub.2 O+H→N.sub.2 +OH.

Kramlich et al (Combustion and Flame 77:p. 375-384, 1989) have madeexperiments in order to study the N₂ O formation and destruction in atunnel furnace, which was fired on either natural gas or oil.Nitrogen-containing compounds, such as HCN and acetonitrile, were dopedinto the flow. According to Kramlich et al maximum N₂ O emissions ofabout 245 ppm occurred at 977°-1027° C. for HCN addition and of about150 ppm at 1127°-1177° C. for acetonitrile addition. The study alsoshowed that N₂ O concentration was reduced from 240 ppm to 10 ppm byincreasing the tunnel furnace temperature to over 1200° C. during HCNinjection into the furnace or to over 1300° C. during acetonitrileinjection, i.e. relatively high temperatures were needed according tothis study.

Kramlich et al also studied the influence of NO_(x) control on N₂ Oemissions. Especially the reburning of a portion of the fuel by fuelstaging in the tunnel furnace was studied. In reburning, a portion ofthe fuel is injected after the main flame zone, which drives the overallstoichiometry to a fuel-rich value. After a certain time in thefuel-rich zone, air is added to fully burn out any remaining fuel.Kramlich et al discovered that reburning of coal in a second stageincreases N₂ O emissions whereas reburning of natural gas in the furnaceexerts an opposite influence to that of coal and destroys N₂ O.

It is an object of the present invention to provide a simple andeconomical method and apparatus for the reduction of N₂ O emissions fromatmospheric and pressurized circulating and bubbling fluidized bedboilers.

It is further an object of the present invention to provide a method andapparatus for creating favorable conditions for the destruction ofnitrous oxides N₂ O contained in flue gases discharged from fluidizedbed combustors.

It is still further an object of the present invention to provide amethod for reduction of N₂ O in flue gases which can easily beretrofitted into existing fluidized bed combustion systems withoutinterfering with existing processes.

In accordance with the present invention there is provided a method ofreducing emissions of N₂ O in flue gases from the combustion of nitrogencontaining fuels in a fluidized bed reactor. A first combustion stage isarranged in a fluidized bed of particles. Fuel and an excess of anoxygen-containing gas at an air coefficient>1 may be introduced into afirst combustion stage for combustion of the fuel (i.e.oxygen-containing gas may be injected into the first combustion stage inan amount to generate flue gases containing residual oxygen). Atemperature of about 700° C.-900° C. is maintained in the firstcombustion stage. The flue gases containing residual oxygen are conveyedfrom the first combustion stage into a flue gas passage. An additiveselected from a group of chemical compounds able to form hydrogen (H)radicals is injected into the flue gas passage in order to generatesufficient quantities of hydrogen radicals to promote the reduction ofN₂ O in the flue gases. Preferably the additive injected is combusted toprovide combustion heat for raising the temperature of the flue gaspassage to >900° C., preferably to about 950°-1100° C. The group ofadditives able to form hydrogen radicals comprise compounds such asalcohol or natural gas, or other hydrocarbon gases such as liquefiedpetroleum gas or gasifier or pyrolyser gas, or oil. A good mixingbetween the flue gas and the formed hydrogen radicals is provided byinjecting the additive at a location where a good mixing is easilyarranged or is already prevailing in the flue gas flow. Good mixingfacilitates the reactions between N₂ O and H radicals. The amount ofadditive injected is adapted to the amount of N₂ O in the flue gases.

The present invention is especially applicable when combusting solidfuels or waste materials in fluidized bed combustors at temperaturesbelow 900° C. The solid fuel or waste is introduced into the fluidizedbed where--due to good mixing with the fluidized particles--it almostimmediately reaches the bed temperature and is combusted. Temperaturesin fluidized beds are normally between 700°-900° C. which gives optimalconditions for the combustion itself and, e.g., sulphur reduction in theflue gases. NO formation is low due to the relatively low combustiontemperature, but N₂ O may be formed.

In circulating fluidized beds the velocity of the fluidizing air is highenough to entrain a considerable amount of the bed particles out fromthe combustion chamber with the flue gases. The particles entrained areseparated from the flue gases and recycled to the combustion chamberthrough a recycling duct. The circulation of particles from thecombustion chamber through the particle recycling path back to thecombustion chamber brings about a uniform temperature in the entiresystem which leads to more efficient combustion and longer residencetimes in the system as well as improved sulphur capture from flue gases.

Unfortunately N₂ O formation seems to be facilitated by the lowtemperatures used in both bubbling and circulating fluidized beds.According to the present invention the N₂ O concentration in the fluegases can be decreased by the injection of an additive capable forforming hydrogen radicals at the flue gas temperature and/or by slightlyincreasing the temperature of the flue gases.

The types of additives (e.g. additional fuels) which can be injectedinto the flue gas flow in order to reduce N₂ O concentration include:

natural gas or methane,

liquefied petroleum gas,

oil,

alcohol, e.g. methanol or ethanol,

gas from a pyrolyser or gasifier, or

any gaseous, liquid or solid fuel, having a hydrogen component, and aheat value of at least 1 MJ/kg.

Gases may be introduced through gas inlet nozzles without any carriermedium, or with an oxygen containing gas. Oil or fine solid fuel may beintroduced with carrier gas such as air or recycled flue gas.

The additives injected into the flue gases are preferably injected at alocation separate from the first combustion stage in order not tointerfere with reactions taking place there. Preferably the additivesshould not be injected so that they significantly increase thetemperature of the fluidized bed particles.

In order to ensure effective reduction of N₂ O the additive should beinjected at a location where the whole flue gas flow can easily beaffected by the introduction of the additive. The temperature of thewhole flue gas flow should be increased and/or hydrogen radicals formedshould come into contact with the whole flue gas flow in order toachieve a maximum reduction of N₂ O.

The additive or additional fuel may be injected into the followinglocations:

a section of the fluidized bed combustor, or elsewhere, where beddensity is less than 200 kg/m³,

a duct between the combustion chamber and a cyclone or other gasparticle separator,

a cyclone or other gas particle separator itself, in any number ofconfigurations,

ducts between two cyclones or other gas particle separators, orcombination thereof connected in series,

any location in the backpass after the combustor and before a stack orgas turbine, or

any external postcombustor for N₂ O reduction.

By introducing additional fuel, such as natural gas, in the flue gaspassage in front of the convection section where the temperature of theflue gas still is high, only a relatively small amount of additionalfuel is needed to increase the temperature of the flue gas flow to over900° C. (i.e. 1652° F.)

A cyclone separator may provide very good mixing of flue gases and anyadditive introduced therein. It may, however, be more preferable toincrease the temperature of the flue gases at a location downstream ofthe particle separator (at least in circulating fluidized bed systems)in order not to increase the temperature of fluidized bed particles andinterfere negatively with sulphur capture (which is optimal at lowertemperatures).

The introduction of additional fuel into the flue gases may beadvantageously used to increase the temperature of the flue gasesupstream of superheaters, thereby ensuring sufficient heating capacity.The fuel may be added into a convention section immediately before thesuperheaters. The introduction of combustible additives may also be usedto simultaneously increase the temperature of gas in a combustionchamber or so called topping combustor connected to a gas turbine.

When additional fuel is introduced into the flue gas flow before theconvection section, the temperature of the flue gas has to be onlymoderately increased from temperatures of about 700°-900° C. (i.e. about1292°-1652° F. and after) to temperatures of about 910°-1100° C. (i.e.about 1670°-2012° F.) i.e. a temperature increase of about only 10°-250°C. is enough, because of the presence of particles (e.g. calcinedlimestone) from the fluidized bed. If the flue gases pass through aconvection section, their temperature is substantially reduced.Therefore, if the N₂ O reduction is performed after the convectionsection, the temperature of the flue gases must be raised about200°-700° C. (i.e. about 392°-1292° F.) in order to get it into the910°-1100° C. (1670°-2012° F.) range. Therefore, the amount of fuelnecessarily added after a convection section is greater than the amountnecessary before a convection section.

By using this process according to the invention to increase temperatureand/or H radical concentration in the flue gases it may be possible toreduce the total amount of N₂ O by 10-99%, normally about 50%, andpreferably about 50-90%. The mass flow of the additive is defined by thepercentage of N₂ O reduction required and the initial concentration ofN₂ O.

In addition to the additive (e.g. additional fuel) injected, a suitableamount of oxidizing agent may in some cases be injected into the N₂O-containing flue gas before, at the same location, or after fuelinjection point to guarantee efficient firing.

The present invention provides a method, which brings about conditionsfavorable to reduction of N₂ O in flue gases in fluidized bedcombustors, and thus a simple way of reducing N₂ O emissions in fluegases. The new method can easily be utilized in existing fluidized bedreactor systems by introducing an additive into flue gas ducts, beforestack or gas turbines or into external postcombustors. There is no needto interfere with the primary combustion process or the reactions takingplace in the combustion chamber itself. Surprisingly, only a very slightincrease in temperature may be needed for the reduction of N₂ O in theflue gases. Prior art studies indicate destruction of N₂ O in thefurnace itself and at much higher temperatures. The increasedtemperature helps to promote destruction of N₂ O not only by H-radicalsin the gas phase but also the heterogenous reaction between N₂ O andcalcined limestone. Prior art studies show that N₂ O formation reaches amaximum at the very temperatures at which N₂ O is destroyed according tothe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below in more detail by reference toillustrative embodiments represented in the drawings in which:

FIG. 1 is a schematic drawing of an exemplary circulating fluidized bedsystem for reducing N₂ O in accordance with the present invention;

FIG. 2 is a schematic drawing of another exemplary embodiment; and

FIG. 3 is a schematic drawing of still another exemplary embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is shown in FIG. 1,where solid material is combusted in a circulating fluidized bed reactor10. The reactor includes a combustion chamber 12 containing a fluidizedbed of particles 13 with inlets 14, 16 for solid fuel material, andtypically other solid material such as lime or limestone for thereduction of SO₂ in flue gases. Fluidizing air is led into thecombustion chamber through a bottom plate 18 from a windbox 19. The airis lead into the reactor at nearly an atmospheric pressure at a flowrate high enough to fluidize the bed and entrain a portion of the solidparticles.

The combustion chamber has an outlet 20 for flue gases containingentrained solid particles. The flue gases are led to a cyclone separator22 where solid bed particles are separated from the gases. The cleanedgas is discharged through a gas outlet opening duct 24 and the particlesseparated from the gas are led downwards through a vertical return duct26 back into the lower part of the combustion chamber. The return ductforms a bend 28 at its lower end in front of the inlet to the combustionchamber.

The cleaned gas is led via the gas outlet opening 24 into a gas passage30 which connects the fluidized bed reactor with a convection section32. A superheater 34 is arranged in the gas inlet zone of the convectionsection and other heat transfer surfaces 36, downstream from thesuperheater. Gas outlet 38 is arranged in the bottom part of theconvection section.

An additive inlet 40 for hydrogen radical providing additive is arrangedin the gas passage 30 connecting the cyclone with the conventionsection. The inlet for additive is disposed in the gas passage at alocation close to the cyclone gas outlet opening 24.

In operation, combustion is effected in a first combustion stage in thecombustion chamber at a relatively low temperature (e.g. when combustingcoal at about 850° C.). At this temperature a low No₂ combustion isachieved and a maximum sulphur capture with lime occurs. Flue gasescontaining residual oxygen and N₂ O and entrained bed particles aredischarged through the gas outlet 20 into the cyclone 22. Bed particlescontaining unreacted lime for sulphur capture are separated from theflue gases in the cyclone and recycled into the combustion chamber.

An additive, such as natural gas, is injected into the still-hot fluegas, e.g. in the duct 30 through the additive inlet 40 (immediatelyafter cyclone 22). The natural gas to some extent provides hydrogenradicals already at the flue gas temperature, but due to the residualoxygen content in the flue gases natural gas is combusted when enteringthe flue gas passage 30, thus increasing the flue gas temperature in theflue gas passage to a still more favorable level when consideringhydrogen radical formation and N₂ O reduction to N₂. Alternatively, oradditionally, O₂ containing gas may be added in inlet 40 mixed with theadditive.

The introduction of additive may additionally or alternatively beaccomplished through an inlet 42, shown as a broken line in FIG. 1, inthe short duct 21 connecting the combustion chamber 12 and the cyclone22. This inlet 42 may be used especially if the particle content of theflue gases discharged from the combustion chamber, is small. It isfurther possible to arrange an additive inlet 44 directly into thecyclone 22, into a particle lean zone. The advantage of this arrangementis inherently good mixing between flue gases and introduced additive inthe gas vortex in the cyclone.

The additive may also or alternatively be injected into the conventionsection through an inlet 46 arranged immediately upstream of thesuperheater 34. This arrangement is advantageous if there are problemsin getting enough superheating steam.

Another embodiment of the present invention is shown in FIG. 2. In thisembodiment heat exchange tubes 38, e.g. a few rows of screen tubes, aredisposed in a gas duct 30 after a cyclone, but before the duct enlargensinto a convection section 32.

All optimal location for an additive inlet 40 seems often to beimmediately after the screen tubes 38. Normally the screen tubes arewater cooled, but can in some applications be steam or air cooled. Hightemperatures in the gas duct can cause problems if the tubes are air orsteam cooled. The water tubes can be connected to other water/steamsystems, e.g. the cooling system of a cooled cyclone, in the fluidizedbed reactor. If air cooled tubes are used, the heated air can be used ascombustion air. The heated air can also be used to inject hydrogenradical providing additive into the gas duct.

A heat exchanger arranged upstreams of the injection of hydrogen radicalproviding additive is advantageous for flattening the gas velocityprofile in the gas duct. This is useful because the flue gas from thecyclone exit may have a skewed velocity profile.

The heat exchanger is further useful to control the flue gas temperatureso that the additive is injected at the optimum temperature for maximumeffectiveness. With the heat exchanger the temperature can be regulatedto an optimal level. For each additive there is an optimal temperaturefor maximum efficiency.

Another embodiment of the present invention is shown in FIG. 3, wheresolid material is combusted in a pressurized circulating fluidized bedreactor 50. The pressurized flue gas is led through a cyclone 52, forseparating particles from the gas, and a convection section 54 into aparticle filter 56 for cleaning the pressurized flue gases. The cleanedflue gas is led into a combustion chamber 58 immediately upstream of agas turbine 60, where the flue gas is expanded. In the combustionchamber 58 reduction of N₂ O is accomplished by introducing additionalfuel into the flue gas through inlet 62 and combusting the fuel tosimultaneously increase the temperature of the flue gas.

In all embodiments it is necessary to adjust the amount of additive (andoxygen) introduced depending upon the type of additive, fuel, fluidizedbed reactor, position of injection, and a wide variety of other factors.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A process for the reduction of nitrous side inthe effluent from the combustion of a carbonaceous fuel, the processcomprising:(a) forming an effluent in a circulating fluidized bedboiler; and (b) raising the temperature of said effluent to atemperature of at least about 1742° F., when said effluent is at atemperature below about 1742° F.
 2. The process of claim 1 whichcomprises raising the temperature of effluent to a temperature of atleast about 1850° F.
 3. The process of claim 1 wherein the temperatureof the effluent is raised by means of a heating means.
 4. The process ofclaim 1 which further comprises introducing a source of hydrogenradicals into the effluent at a location at or near that where theeffluent temperature is raised.
 5. The process of claim 4 wherein saidsource of hydrogen radicals comprises a hydrocarbon.
 6. The process ofclaim 5 wherein said hydrocarbon is alcohol.
 7. A boiler consisting of acirculating fluidized bed boiler comprising an effluent flow path inwhich is dispose a heating means for raising the effluent temperature toat least about 1742° F., said heating means located where the effluenttemperature is less than about 1742° F.
 8. The boiler of claim 7 whichfurther comprises an introducing means for introducing a source ofhydrogen radicals into the effluent.
 9. A boiler as recited in claim 8wherein said introducing means comprises an injector.
 10. The boiler ofclaim 8 wherein said introducing means is disposed in the boiler at ornear said heating means.
 11. The process of claim 1 wherein step (b) ispracticed by providing sufficient oxygen in the effluent to effectcombustion, and by adding a gaseous, liquid, or solid fuel having ahydrogen component, and a heat value of at least 1 MJ/kg.